how to export each animation separately into gLTF 2.0?
From the glTF addon docs:

glTF allows multiple animations per file, with animations targeted to particular objects at time of export. To ensure that an animation is included, either (a) make it the active Action on the object, (b) create a single-strip NLA track, or (c) stash the action.

You will need to create the NLA track or stash the action on each armature that you want that action to be available for, unless Godot has some ways of re-assigning the animations later.